This invention is in the field of ignition systems for fuel burning engines and in particular in such ignition systems which have both a capacitor and an inductive winding of an ignition transformer cyclically charged and discharged in discharge aiding mode, and more particularly wherein such system produces a high velocity igniter arc.
The principal prior art ignition systems may be categorized into three groups. The first category of such prior art systems, referred to as the Kettering system, uses a capacitor in series with a primary winding of an ignition transformer wherein the capacitor is short-circuited by a timer so as to permit the primary winding to be charged by a DC source. The timer then removes the short circuit from the capacitor to permit the charged winding to discharge into the capacitor so as to create a single ringing circuit component, used to fire an igniter.
The second category of such prior art systems, referred to as a capacitive discharge system, also has a capacitor in series with an ignition transformer winding. Controlled by an appropriate timer, the capacitor is charged, generally by a higher DC voltage than in the Kettering system, such higher DC voltage being generated in the system. The timer then enables the charged capacitor to discharge into the transformer winding also creating a single ringing current component of somewhat higher voltage peak than the Kettering system to fire an igniter.
The third category of such prior art systems involves the use of a generated AC wave by such prior art system and attempts to apply such generated wave either to an ignition transformer or directly to a distributor in order to fire an igniter.
With respect to the first category, or Kettering prior art system, the main problem lies in the fact that the system attempts to precharge an inductor using a DC source in anticipation of an igniter firing cycle. It is well-known that an inductor energized by DC cannot charge to its full current level in a short period of time, and therefore, cannot rapidly produce an induced voltage therein. Hence, only a portion of the maximum current quantity can be made to flow through the primary winding during the charging mode, with consequent nonuse of the full energy storage capability of such primary winding, and therefore, loss of electrical power delivery capability to fire the igniter is experienced.
The current conduction through an inductor powered by a DC source, such as a battery, when switched on by the timer may be expressed as: ##EQU1## where i is current at any instant of time, t is time, V.sub.dc is the voltage provided by the DC source, R is the circuit resistance of the inductor and DC source, L is the inductance of the primary winding and R/L is the time constant of the circuit.
From such equation, it can be seen that when t=0, i=0, and when t approaches infinity, i approaches the value of V.sub.dc /R. Based on typical values of L and R, it would take about 100 milliseconds for the primary winding to be almost fully charged, and a typical primary winding charging period is generally not greater than 5 milliseconds.
It is, therefore, obvious that the use of a DC source to charge the primary winding of the Kettering or first category of ignition systems is self-defeating in that possibly no more than half the inductor's charge capacity can be effectively utilized.
With respect to the second category or capacitive discharge system, a like result, with very little improvement over the Kettering system, is realized.
In such second category system, the higher DC voltage to precharge the capacitor is obtained by using an electronic oscillator to generate a higher AC voltage which is then converted to DC by rectification and filtering. The higher DC voltage is controlled by a timer to precharge the capacitor and then discharge the capacitor into the ignition transformer winding to fire an igniter. If one keeps in mind that a charged capacitor is just like a DC source, then one can apply the foregoing equation which defines current in the transformer winding. Although the value of V.sub.dc representing the charged capacitor will be higher than in the case of the Kettering system, one must not lose sight of the fact that the energy content of a charged capacitor is limited by the capacitance and hence its ability to deliver current for an extended time period is limited. Hence, although a higher peak single ringing cycle will result due to the charge from the capacitor being dumped into the transformer winding, the single ringing period will be substantially shortened compared with the single ringing period of the Kettering system.
Since energy is a function of the product of power and time, the advantage of the capacitive discharge system over the Kettering system is minimized due to the lesser amount of time during which energy is present to fire the igniter.
With respect to the third category of prior art ignition systems or the AC systems, the major problem resides in the inability of the prior art to recognize how to transfer the power from the AC generator to the load, the load generally being a transformer. Consequently, although such system might basically be able to provide AC power over longer periods of time, these systems suffer from the lack of technique in effectively tranferring such power and particularly providing higher current levels to the load.
The need for such higher current levels has been repeatedly stated in periodicals and patents written by those in the automotive manufacturing industry and in the automotive fuel-producing industry such as Texaco. Such periodicals or patents generally show a high power AC rectangular wave generator employing a transformer wherein one of the windings thereof is used to saturate the transformer core by employing a DC source connected to that winding, so as to prevent the generator from producing power. A timer, coupled to such winding, enables the core to go out saturation, and ostensibly enables the generator to provide AC power by magnetic induction through a high voltage winding of the transformer to an igniter load.
The basic problem with such generator resides in the high impedance experienced in the electronic circuit of the generator where the transistors are located, when under actual load conditions such as when the igniter is attempting to arc. Reflected impedance of the high voltage winding into the lower voltage winding to which the transistors are connected plus the self-impedance of such lower voltage winding would severely limit the current circulating in the collector-emitter circuits, and consequently would result in a lowered voltage and severely reduced current levels deliverable to the actual igniter. Thus, not only is the voltage across the so-called high voltage winding of such prior art AC system lower than expected, but the required higher current level for feeding the igniter in order to overcome high pressure fuel-flow across the igniter base, and in particular where the air-to-fuel ratio is in the order of 18 to 1 or greater (lean-burn engines), is not available.
Additionally, such prior art AC systems are inhibited from rapid duty cycling of their AC generator principally by magnetically saturating the generator's transformer core to inhibit oscillations. Sight is lost of the fact that the DC current used to saturate such core results in a comparatively long time for the core to reach saturation (see formula above), and hence slows up the cycling of the generator between its operative and quiescent mode. As a result, the prior art AC systems provide triangular-shaped current waveforms which inherently have slow rates of change in their waveforms as a function of time and therefore result in a reduced induced voltage in the high voltage winding, inasmuch as by Faraday's law of induction, such reduced voltage is a function of the rate of change of current. It can be appreciated that if, for example, the prior art could have overcome the above problems residual in their AC source and could provide a current waveform output with a fast rate of change, such as one approaching a rectangular waveshape, at least the output voltage of such generator would be increased. However, the problem of being able to deliver higher currents to the load would still remain unsolved.
Accordingly, neither the Kettering, capacitive discharge, nor AC system is capable of delivery of sufficient quantities of energy to fire an igniter, in order to enable the igniter to cause all fuel in an engine cylinder to be consumed and not wasted by failure of the ignition arc to burn same.
A further disadvantage of prior art ignition systems is that they cannot charge the inductor or transformer winding and the capacitor in a way so that discharge currents therefrom are additive and aid each other.
A still further disadvantage of the prior art systems is their inability to deliver sufficient energy to fire an igniter for extended periods of time.
Yet a further disadvantage of the prior art systems is their inability to deliver more than one ringing cycle during an igniter firing period.
Yet another disadvantage of the precharged inductor or capacitor prior art systems is their inability to rapidly charge the inductor due to use of DC power, with attendant inability to deliver sufficient energy to fire an igniter so as to effectively cause all the fuel to burn during an igniter firing period.
Yet another important disadvantage of any prior art system is the inability of the system to accelerate the arc luminous particles to such high velocity so that such arc can adequately overcome internal engine and fuelflow pressures. Such prior art systems are therefore unable to use an igniter that develops long arc lengths between its electrodes. Such deficiency results in initation of a small fuel ignited nodule during the initial ignition period which is insufficient in mass and area to cause all fuel in a cylinder to be consumed and not wasted.
Other disadvantages with such prior art systems reside in their complexity due to the need of a large quantity of electronic components which also gives rise to unreliability as well as high cost of production.
Exemplary of prior art systems is U.S. Pat. No. 3,714,507 which is a capacitive discharge system. A charge retention storage capacitor is charged by a relatively high DC voltage source, and the charge from the charge capacitor is discharged through an ignition transformer primary winding by utilizing a silicon controlled rectifier switch. Another capacitor across the primary winding is selected of such value so as to suppress electromagnetic interference due to discharge of the storage capacitor.
Another example of prior art is U.S. Pat. No. 3,312,860 which operates on a similar principle to that of U.S. Pat. No. 3,714,507, except that its high voltage DC power source is of a different design.
Still another example of the prior art is U.S. Pat. No. 3,972,315 which utilizes two ignition transformer primary windings. One of such windings is energized by the discharge of a precharged capacitor from a DC source, whereas the other of these primary windings has a discharge current passing therethrough to combine with the capacitive discharge into the first named primary winding. This would be the principle of operation if the system were operative, but such system is precluded from operation by a hard-wire short circuit across the second named primary winding.
All of these exemplary systems miss the major point of technology of not utilizing rectangular or other AC power to feed the ignition transformer primary winding, and to feed such components as are connected in the primary winding circuit with AC power, and thus such systems cannot obtain the extremely high energy levels that would otherwise be possible when exciting the ignition transformer primary circuit components with AC power.